1. Field of the Invention
The present invention relates to a serial data transmission apparatus which transmits data in synchronization with clock signals.
2. Description of the Prior Art
Various apparatuses are known for transmitting serial data in synchronization with clock signals. For example, a multi-bit light emitting diode (LED) eraser unit is used in an electrophotographic copying machine for erasing an area not used for printing in an electrostatic latent image formed on a photoconductor. Serial data transmission may be used for turning on or off each LED in the unit for simplifying a harness provided for signal lines, especially for an eraser having thirty or more LEDs.
When an LED eraser unit adopts serial data transmission, serial signals such as data on turning on or off each LED, clock signals, latch signals and/or the like are sent from a controller for erasing to the eraser unit. However, because signal lines are long, there are many noise sources around the signal lines. Then, noises are liable to be added to the serial signals, to cause malfunctions of the eraser unit.
In order to prevent erroneous transmission of serial data, it is proposed to detect an error by communication between a controller and an option unit such as an LED eraser or the like, and to send the signal again after processing the erroneous signal. For example, a controller disclosed in Japanese Patent Publication 4-41400/1992 detects if an answer is received from an option unit within a prescribed time after a signal is sent to the option unit. If it does not receive the answer, it decides that a transmission error happened and it sends the same signal again. A controller disclosed in Japanese Patent Publication 4-41399/1992 detects if an answer is received from an option after a signal is sent to the option and decides that an error happens when it does not receive the answer after a number of prescribed times.
However, a unit such as an LED eraser unit is required to be controlled in real time. Then, if noises are induced in transmission lines between the controller and the unit, the unit does not operate normally. For example, an LED eraser unit has to be controlled in synchronization with the rotation of the photoconductor drum. Then, if noises are induced in signals, or even in one signal such as a clock signal, the timing, for example, of turning on LEDs is shifted from the proper timing, and quality of an image formed on a paper may be deteriorated.